An extremely curved relativistic jet in PKS

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1 An extremely curved relativistic jet in PKS Tuomas Savolainen 1 Kaj Wiik 1,2, Esko Valtaoja 1 & Merja Tornikoski 3 1 Tuorla Observatory, Finland 2 Institute of Space and Astronautical Science, Japan 3 Metsähovi Radio Observatory, Finland

2 A description of the original project On May 2001 we made multi-frequency polarimetric observations of four high peaking GPS-sources using the VLBA. The aim was to study whether these sources with peak frequencies around GHz belong to the same class of objects as classical GPS-sources peaking around 1 GHz, or whether they form a distinct group. The sample included PKS , PKS , PKS and PKS Frequencies used were 2, 5, 8, 15, 22 and 43 GHz.

3 PKS (OX 161) A quasar type GPS source with ν max ~ 8-10 GHz z = Compact in VLA images without any extended structure (Murphy et al. 1993) Core-jet morphology in VLBI scales (e.g. Kellermann et al. 1998) β app = 1.8±1.4 (Kellermann et al. 2004) Variable at cm-wavelengths (M. Aller, private communication) Tornikoski et al. 2001

4 The multi-frequency appearance of PKS

5 2 GHz 5 GHz 8 GHz The multi-frequency appearance of 43 GHz PKS GHz 15 GHz

6 -27º +123º 210º Largest ever observed P.A. in an extragalactic jet!

7 The trajectory Self-calibrated visibilities were model-fitted in DIFMAP with circular Gaussian model components We have fitted different helical trajectories to component positions the figure shows our best fitting model (χ 2 =1.7): a helix on a surface of a cone with linearly increasing pitch angle

10 The peak in the Doppler factor around phase angle of 10º is almost coincident with apparent brightening at 0.28 mas from the core Observed β app = 1.8±1.4 (Kellermann et al. 2004) is consistent with the model If the bending is due to helical trajectory, the helix should be quickly opening and already asymptotically approaching a straight line in the scale of 15 mas

11 Reason for bending

12 Density gradients? The observed bend > 180º. This excludes scenarios like: deflection by a massive cloud in the ISM bending by a uniform density gradient Large misalignment angle in PKS was explained by Homan et al. (2002) with a jet bending after it departs the host galaxy (due to a density gradient or ram pressure by intracluster winds). In PKS bending takes place well within 15 mas corresponding to 120 pc. Assuming typical size of elliptical hosts of radio galaxies (~30 kpc) would imply a viewing angle smaller than 0.2º, if the jet bent after departing the host galaxy. This would put our line of sight inside the jet opening angle, which is not observed. Thus, the jet bends before it reaches the outskirts of the host galaxy.

13 Jet precession? Simple model: a ballistic jet with a precessing nozzle Easily testable with further observations: The P.A. of the most compact part of the jet should change with time The components should follow straight not curved trajectories We tried to fit such a model to the our data

14 A precessing ballistic jet fits rather well to the component locations, but we could not (yet?) find a solution with plausible viewing angles.

15 MHD instabilities? Bending could also be due to growing MHD instabilities (pressure driven, Kelvin- Helmholtz or current driven; see e.g. Hardee 1987, 2003) Difficult to measure the wave speed, the wavelength or the growth rate of the disturbance with the current data

16 Future observations With further VLBI observations, we have a good chance to determine the 3-D trajectory of the source: Flux density, jet opening angle and component velocity all depend on the viewing angle. By measuring them as a function of distance along the jet, we can try to fit for viewing angle (and its changes) and find out the 3-D structure High frequency VLBI observations, 2-3 times per year covering a few years, should do the job

17 Summary PKS shows the largest apparent bend ever observed in an extragalactic jet Bending is consistent with an intrinsic helical trajectory The jet bends before it reaches the outskirts of the host galaxy (unlike in PKS ) MHD instabilities or jet precession could be viable explanations. Further study is needed here. With VLBI monitoring, the 3-D trajectory of this jet should be tractable PKS may prove to be a good test bench for studying jet bending scenarios

arxiv:astro-ph/ v2 4 May 2006

arxiv:astro-ph/ v2 4 May 2006 Accepted for publication in The Astrophysical Journal. Preprint typeset using L A TEX style emulateapj v. 4/21/5 AN EXTREMELY CURVED RELATIVISTIC JET IN PKS 2136+141 Tuomas Savolainen, Kaj Wiik 1, Esko